Pyrolysis reactors may be utilized to pyrolyzing or cracking hydrocarbons. These pyrolysis reactors may include regenerative processes that perform cycles, which are either symmetric (same chemistry or reaction in both directions) or asymmetric (chemistry or reaction changes with steps in the cycle). Examples of these reactors and the associated processes are described in U.S. Pat. Nos. 2,319,679; 2,678,339; 2,692,819; 3,024,094; 3,093,697; and 7,943,808. As one of the steps in the cycle, combustion can be used for regenerating reactors to perform cyclic, high temperature chemistry.
The process typically involves a heating step (e.g., regeneration step) and a pyrolysis step in different portions of a cycle. The heating step includes exothermic reactions, e.g., by conducting fuel and oxidant to a reaction zone, combusting the fuel and oxidant, and then conducting the combustion products away from the reaction zone. During the pyrolysis step, a feed containing hydrocarbons is conducted through the reaction zone, thereby pyrolysing the hydrocarbons in the feed and conveying heat from a reactor bed or other source. Some regenerative pyrolysis reactors deliver fuel and/or oxidant directly to the combustion zone without having that stream pass through reactor beds that preheat the stream. The fuel and/or oxidant is typically introduced via nozzles, distributors, or burners that penetrate the reactor using means generally perpendicular to the reaction flow direction and usually through the reactor vessel side wall. For example, during the heating step in a conventional Wulff cracking furnace, air flows axially through the regenerative bodies, and fuel is introduced via nozzles that penetrate the side of the reactor, to combine with air (combusting and releasing heat) in an open region between regenerative bodies.
In these systems, the reactor may include various components to manage the flow of streams (e.g., process-flow components) and may include insulating materials (e.g., insulation components). These insulating components typically involve materials, such as alumina or zirconia. As an example, U.S. Pat. No. 2,823,027 teaches the use of SiC, alumina or zirconia reactor components surrounded by a lining of insulation inside a steel vessel. This reference describes that the use of tile, which are formed of a material selected from the group which includes silicon carbide and aluminum oxide. These materials have a relatively high heat storage and heat conductive capacity and can be subjected to temperatures in the neighborhood of 3000° F. (1649° C.) for long periods without damage. Other refractories may include magnesium oxide, zirconium oxide, or high temperature porcelain. However, the high temperatures and process stresses can exceed the long term viability of most conventional component materials, including conventional refractory ceramics. In addition to component physical and thermal performance considerations, component chemical inertness and crystalline stability are also considerations that should be considered.
As the presence of carbon from the hydrocarbon in a feed and potential presence of oxygen present combustion streams may result in premature ceramic corrosion, chemical stability may also present certain challenges. Many prior art ceramic materials that are relatively inert at lower temperatures become susceptible to chemical degradation, ceramic corrosion, and/or crystalline alteration at higher temperatures, leading to premature degradation, and/or process interference such as by generation of unacceptable levels of contaminants in the process. Exemplary chemically and/or thermally unstable ceramics include, but are not limited to certain borides, carbides, and nitrides. Similarly, while zirconia is commonly used in certain refractory ceramics, it undergoes a crystalline change between moderately high temperatures and severely high temperatures in the way its atoms are stacked (polymorphic transformation).
Accordingly, the configuration of reactor components, such as process-flow components and insulation components, within pyrolysis reactors may be utilized to enhance the operation of a pyrolysis reactor, while providing thermal and chemical stability. That is, a ceramic composition and configuration of ceramic materials that resists or avoids carbon permeation, carburization, and/or oxide-carbide corrosion is needed. The desired materials should concurrently provide and maintain the needed structural integrity, crystalline stability, relatively high heat transfer capability, and chemical inertness required for large scale, commercial applications, particularly with respect to hydrocarbon pyrolysis.